Electrosurgical return electrode monitor

ABSTRACT

The present disclosure provides a method for determining the probability of a patient burn under a return electrode in a monopolar electrosurgical system comprising calculating a heating factor adjacent the return electrode utilizing a first algorithm, calculating a cooling factor adjacent the return electrode utilizing a second algorithm, subtracting the calculated cooling factor from the calculated heating factor to obtain a difference value, comparing the difference value to a threshold value, and adjusting the power dependent on the relationship of the difference value to the threshold value.

BACKGROUND

[0001] 1. Technical Field

[0002] This application relates to an apparatus and method fordetermining the probability of patient burn during electrosurgery, andmore particularly to determining the probability of patient burn under areturn electrode in a monopolar electrosurgical system.

[0003] 2. Background of Related Art

[0004] During electrosurgery, a source or active electrode deliversenergy, such as radio frequency energy, to the patient and a returnelectrode carries the current back to the electrosurgical generator. Inmonopolar electrosurgery, the source electrode is typically thehand-held instrument placed by the surgeon at the surgical site and thehigh current density flow at this electrode creates the desired surgicaleffect of cutting or coagulating tissue. The patient return electrode isplaced at a remote site from the source electrode and is typically inthe form of a pad adhesively adhered to the patient.

[0005] The return electrode has a large patient contact surface area tominimize heating at that site since the smaller the surface area, thegreater the current density and the greater the intensity of the heat.That is, the area of the return electrode that is adhered to the patientis important because it is the current density of the electrical signalthat heats the tissue. A larger surface contact area is desirable toreduce heat intensity. Return electrodes are sized based on assumptionsof the maximum current seen in surgery and the duty cycle (thepercentage of time the generator is on) during the procedure. The firsttypes of return electrodes were in the form of large metal platescovered with conductive jelly. Later, adhesive electrodes were developedwith a single metal foil covered with conductive jelly or conductiveadhesive. However, one problem with these adhesive electrodes was thatif a portion peeled from the patient, the contact area of the electrodewith the patient decreased, thereby increasing the current density atthe adhered portion and in turn increasing the heat applied to thetissue. This risked burning the patients in the area under the adheredportion of the return electrode if the tissue was heated beyond thepoint where the circulation could cool the skin.

[0006] To address this problem, split return electrodes and hardwarecircuits, generically called Return Electrode Contact Quality Monitors(RECQMs), were developed. These split electrodes consist of two separateconductive foils. The hardware circuit uses an AC signal between the twoelectrode halves to measure the impedance therebetween. This impedancemeasurement is indicative of how well the return electrode is adhered tothe patient since the impedance between the two halves is directlyrelated to the area of patient contact. That is, if the electrode beginsto peel from the patient, the impedance increases since the contact areaof the electrode decreases. Current RECQMs are designed to sense thischange in impedance so that when the percentage increase in impedanceexceeds a predetermined value or the measured impedance exceeds athreshold level, the electrosurgical generator is shut down to reducethe chances of burning the patient

[0007] Although monitoring circuits in present use are effective, theydo not take into account the amount of time the current is beingdelivered. As new surgical procedures continue to be developed thatutilize higher current and higher duty cycles, increased heating oftissue under the return electrode will occur. It would therefore beadvantageous to design a monitoring circuit which would also factor inthe amount of time the current is being delivered in determining theprobability of a patient burn. Based on this probability determination,an alarm signal can be generated or power supplied from the generatorcan be shut off.

[0008] U.S. Pat. No. 4,657,015 discloses a control device for cuttingoff high frequency current during electrosurgery if the heat buildup inthe body tissue exceeds a predetermined value. In the '015 patent, acontrol electrode is affixed to the body spaced from the activeelectrode and separate from the neutral (return) electrode. The controlelectrode is designed to pick up the voltage existing on the body. Thevoltage signal is squared, integrated over time and compared to areference voltage. The high frequency generator is turned off if thevoltage value exceeds the reference voltage. The '015 patent does noteffectively measure heating under the return electrode since themeasurements are calculated by a separate control electrode. The '105patent even states that the effective surface area of the neutralelectrode is not a factor in the heat calculations. Also, the amount oftime the energy is being applied is not a factor in the heatcalculations. Additionally, the '015 patent uses voltage measurement todetermine overheating of tissue. It is currently believed by theinventors of this application that current measurement provides a moreaccurate parameter because voltage values actually measure the abilityto transfer energy through the tissue while current values measureactual heating of the tissue.

[0009] U.S. Pat. No. 4,741,334 discloses a control circuit intended toreduce burning of tissue. As in the '015 patent, a separate controlelectrode is provided to determine the body voltage. The controlelectrode is spaced from the neutral electrode and functions to detect ahigh frequency body surface voltage. The body surface voltage isconverted into dc voltage by a converter and inputted to a comparatorfor comparison to a reference voltage. The generator is turned off ifthe body voltage exceeds the reference voltage. The '015 patent alsodiscloses a monitor circuit for testing whether the neutral electrode isin good contact with the body surface of the patient. A comparatorcompares the body surface voltage detected by the control electrode witha reference voltage derived from the operational voltage of the surgicaldevice. An audible signal is produced when these voltage values reach apredetermined ratio. Similar to the '015 patent, the '334 patentrequires an additional electrode, measures voltage instead of current todetermine overheating, and does not factor in the amount of time thehigh frequency energy is being applied.

[0010] As noted above, it would be advantageous to provide a monitoringcircuit which effectively determines the probability of overheatingtissue, i.e. the probability of patient burn, by measuring current andfactoring in the time period of energy delivery of energy.

SUMMARY

[0011] The present disclosure provides a method for determining theprobability of a patient burn under a return electrode in a monopolarelectrosurgical system comprising calculating a heating factor adjacentthe return electrode utilizing a first algorithm, calculating a coolingfactor adjacent the return electrode utilizing a second algorithm,subtracting the calculated cooling factor from the calculated heatingfactor to obtain a difference value, comparing the difference value to athreshold value, and adjusting the power dependent on the relationshipof the difference value to the threshold value.

[0012] The step of calculating the cooling factor preferably comprisesthe steps of calculating the off time of the output current to obtain anoff time value and multiplying the off time value by a first constantindicative the body's ability to remove heat. The step of calculatingthe heating factor preferably comprises the steps of multiplying thesquare of the output current by a second constant indicative of themeasured impedance at the return electrode, the second constant beingrepresentative of the adherence of the return electrode to the patient,and multiplying the product by the on time value of the output.

[0013] The method preferably comprises the step of generating an alarmif the difference value exceeds the threshold value. The step ofadjusting the power includes the step of shutting off the power if thedifference value exceeds a second threshold value (a predeterminedvalue) and reducing the power if the difference value is below thesecond threshold value.

[0014] The present disclosure also provides a method for determining theprobability of a patient burn in a monopolar electrosurgical systemcomprising calculating a heating factor adjacent the return electrodeutilizing a first algorithm, calculating a cooling factor adjacent thereturn electrode utilizing a second algorithm, subtracting thecalculated cooling factor from the calculated heating factor to obtain adifference value, comparing the difference value to a threshold value,and generating a warning signal if the difference value exceeds thepredetermined value.

[0015] The first algorithm includes multiplying a current value,obtained by squaring the measured output current, by a constantindicative of the measured impedance at the return electrode and by theon time value of the output current. The second algorithm includesmultiplying the off time of the output current by a constant indicativeby the ability of the body to remove heat.

[0016] The present disclosure further provides an electrosurgicalgenerator for use in a monopolar electrosurgical system having anelectrosurgical tool for treating tissue, a return electrode, and animpedance sensor in electrical communication with the return electrodeto measure impedance of the return electrode. The electrosurgicalgenerator comprises a current sensor for measuring the output currentdelivered by the generator and a microprocessor electrically connectedto the current sensor and the impedance sensor for calculating theheating factor and cooling factor under the return electrode wherein thecalculation of the heating factor is based at least in part on themeasured output current. The generator also includes a controllerelectrically connected to the microprocessor for adjusting the powersupply of the generator in response to the relationship of thecalculated heating and cooling factors. The microprocessor includes afirst algorithm for calculating the heating factor and a secondalgorithm for calculating the cooling factor. The first algorithm isdefined as:

K_(h)I²t_(on)

[0017] wherein K_(h) is the constant related to the contact impedance inOhms of the return electrode, I² is the square of the output current inmilliamps and t_(on) is the time in seconds that the output current isdelivered.

[0018] The second algorithm is defined as:

K_(h)t_(off)

[0019] wherein K_(h) is the constant representative of the time it takesfor the body to cool down in degrees per minute and t_(off) is the timein seconds that the output current is not being delivered.

[0020] The microprocessor also includes an algorithm for subtracting thecooling factor from the heating factor to calculate a difference value,and the generator further comprises a comparator electrically connectedto the microprocessor for comparing the difference value to a thresholdvalue. The comparator is electrically connected to a controller togenerate a first signal indicative of the relationship of the differencevalue and the threshold value. An alarm is electrically connected to thecomparator for generating a warning signal if the difference valueexceeds the threshold value by a predetermined amount. The controllergenerates a shut off signal to terminate power if the difference valueexceeds a predetermined value (the second threshold), the predeterminedvalue being greater than the threshold value, and the controllergenerates a second signal to reduce the power if the difference valueexceeds the threshold value, but is less than the predetermined value.

DETAILED DESCRIPTION OF THE DRAWINGS

[0021] Preferred embodiments of the present disclosure are describedherein with reference to the drawings wherein:

[0022]FIG. 1 is a schematic illustration of a monopolar electrosurgcialsystem;

[0023]FIG. 2 is a schematic block diagram of the electrosurgical systemfor determining the probability of patient burn; and

[0024]FIG. 3 is a flow diagram showing the steps followed forcalculating the probability of patient burn and for controlling theoutput current.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025]FIG. 1 is a schematic illustration of a monopolar electrosurgicalsystem. The surgical instrument for treating tissue at the surgical siteis designated by reference numeral 11. Electrosurgical energy issupplied to instrument 11 by generator 10 via cable 18 to cut,coagulate, etc. tissue. A return electrode, designated by referencenumeral 14, is shown placed under the patient to return the energy fromthe patient back to the patient via wire 12. The return electrode ispreferably in the form of a split pad which is adhesively attached tothe patient's skin.

[0026] The area of the return electrode that adheres to the patient isimportant since it affects the current density of the signal that heatsthe tissue. The smaller the contact area of the return electrode withthe patient's tissue, the greater the current density and the greaterand more concentrated is the heating of tissue. Conversely, the greaterthe contact area of the return electrode, the smaller the currentdensity and the less heating of the tissue. Obviously, the greater theheating of the tissue, the greater the probability of burning thetissue.

[0027]FIG. 2 illustrates a conventional return electrode that is splitto enable the impedance to be measured between the two halves. Theimpedance measurement provides an indication of how well the returnelectrode is adhered to the patient since there is a direct relationshipbetween the impedance and the area of patient contact. If the electrodeis partially peeled from the patient, the impedance increases. This isbecause each portion, e.g. square centimeter, of the electrode pad thattouches the patient has a resistance of its own. All of theseresistances are in parallel, and the resultant equivalent resistance issmaller than any of the individual elements. If any of these parallelresistances are removed because of peeling, the equivalent resistanceincreases slightly. Return Electrode Contact Quality Monitors (RECQMS)utilize an AC signal between the two electrode halves to measureimpedance between them. The diagram of FIG. 2 schematically depicts thisfunction of the RECQM.

[0028] If the total current passed through the return electrode isincreased or the current duty cycle, defined by the percentage of timethe generator is on and the current is applied, is increased, heatingunder the electrode will increase.

[0029] The heating factor of the tissue is a measure of how much heat isdissipated in the tissue. The following equation provides the heatingfactor:

Heating Factor=I ² t _(on)

[0030] where I² equals the current in milliamps, and t_(on) equals thetime the RF generator is on in seconds.

[0031] Thus, the heating factor can be defined as the square of a givencurrent passed through the return electrode attached to a patientmultiplied by the time the current is applied. As is apparent from theequation, if either the current is increased or the on time isincreased, the amount of heat dissipated in the tissue, and thus thechances of patient burn, are increased.

[0032] The foregoing heat factor equation assumes that the area attachedto the patient is unchanged. However, as a practical matter, that areacan change as a portion of the return electrode can become detached fromthe patient. To accommodate for changed surface contact area of thereturn electrode, a constant K_(h) is added to the equation whereK_(h)>=1. This can be modeled by the following equation:

Heating Factor=K _(h) I ² t ² t _(on)

[0033] where K_(h)=1 when the return electrode is fully adhered, andK_(h)>1 if the return electrode is not fully adhered.

[0034] Therefore, as is apparent from the equation, if the surfacecontact area of the return electrode decreases, since K_(h) will begreater than 1, the heating factor will increase. As the surface areadecreases, as explained above, the current density increases and theamount of heating for a given output current also increases.

[0035] Another factor affecting dissipation of heat in the tissue is thetime period the RF energy is applied. The patient's body has the abilityto remove heat from the area under the return electrode by the bloodflow in the capillaries, small arteries and small veins. The more timebetween the applications of RF energy, the greater the heat removalbecause the body will have more time to naturally remove the heat. Thisability to remove heat over a period of time can be modeled by thefollowing equation:

Cooling factor=K _(c) t _(off)

[0036] where K_(c) is a cooling constant dependent on the patient andt_(off) is the time in seconds that the RF energy is off

[0037] During electrosurgery, the RF generator goes on and off manytimes. The conventional Return Electrode Contact Quality Monitor (RECQM)determines if the return electrode is attached to the patient bymeasuring the impedance. However, if a high current or a high duty cycleis utilized, a burn can still occur because the body may not be able toremove the heat fast enough.

[0038] The present disclosure not only measures the current deliveredand the time period the current is delivered, but calculates andcompares the heating and cooling factors in order to measure theprobability of a patient burn.

[0039] More particularly, and with reference to FIG. 2, theelectrosurgical generator includes a microprocessor 26, an adjustablepower supply 22, such as a high voltage supply, for producing RFcurrent, and an RF output stage 24 electrically connected to the powersupply 22 for generating an output voltage and output current fortransmission to the instrument 11. The power supply is adjusted bycontroller 25 dependent on the calculated probability of patient burnwhich is described in detail below.

[0040] The microprocessor 26 has a plurality of input ports. One inputport is in electrical communication with the output current sensor 28which measures the output current of the RF output stage 24 beingtransmitted to the patient. The second input port is electricallyconnected to the current turn on time calculator 30. During the surgicalprocedure, the generator is activated at set or varying time intervals,with intermittent shut down time intervals to allow the tissue tonaturally cool as the patient's blood flow dissipates heat. Thecalculator 30 determines the amount of the time the current is beingsupplied and transmits a signal representative of this calculated timeto the microprocessor 26. The current turn off time calculator 32 sendsa signal to the microprocessor 26 via one of its input portsrepresentative of the time the RP output current has been turned off.

[0041] An algorithm in the microprocessor 26, described in more detailbelow, processes the signals from the current sensor 28 and the timecalculators 30 and 32 in the calculation of the probability of patientburn. The output port of the microprocessor 26 is in electricalcommunication with comparator 34. The calculation of microprocessor 26is compared to threshold values supplied to or present in the comparator34, and if these values are exceeded, an alarm signal is sent togenerate an alarm as a warning to the user. If the threshold values areexceeded, the comparator 34 also sends a power adjustment signal to thecontroller 25 which signals the power supply 22 to either adjust, e.g.reduce the RF output current, or shut off the power supply 22 toterminate the supply of current, dependent on the amount the thresholdis exceeded.

[0042] An impedance sensor 40 forms part of the return electrodecircuit. The sensor 40 measures the impedance between the split pads41,42 of the return electrode pad 44 to determine the degree ofadherence of the electrode pad 44. That is, if a portion of theelectrode pad 44 becomes detached from the patient, the impedance willincrease. The sensor 40 transmits a signal indicative of the measuredimpedance to an input port of the microprocessor 26. The microprocessoralgorithm factors in the impedance measurement in the manner describedbelow.

[0043] Turning now to FIG. 3, the algorithm for calculating theprobability of patient burn (hereinafter “PPB”) and the system foradjusting the RF output current is illustrated in flow diagram. Asshown, output of the RF current is measured and squared, represented byI² in milliamps. The time the RF current is being applied (t_(on)),measured in seconds, is multiplied by the squared current, the formulabeing represented by I²t_(on) to yield a first value.

[0044] As discussed above, the impedance sensor 40 measures theimpedance at the return electrode which is indicative of the degree ofadherence of the return electrode to the patient to thereby provide anadherence constant K_(h). This adherence constant is multiplied byI²t_(on) to calculate the heating factor. Thus, the heating factor iscalculated by the algorithm K_(h)I² t_(on) wherein K_(h) is theadherence constant and K=1 when the return electrode is fully adhered tothe patient and K>1 if the electrode is not fully adhered.

[0045] The cooling factor is calculated by the measured time the currentis not being applied. More specifically, the time off in seconds of theoutput current (t_(off)) is calculated, and multiplied by the coolingconstant K_(c) to calculate the cooling factor as K_(c)t_(off). Thecooling constant K_(c) takes into account that the blood flow in thecapillaries, small arteries and veins of the patient cools the tissueover time. For example, assuming tissue normally cools at 1 degree perminute, since there is some variation, the cooling constant could beconservatively selected as ½ degree per minute. Other constants could beselected depending on the tissue cooling time.

[0046] With continued reference to the flow diagram of FIG. 3, thecooling factor is subtracted from the heating factor by themicroprocessor to determine a value (a “difference value”)representative of the probability of patient burn. (“PPB”). After thiscalculation in the microprocessor, the microprocessor 26 sends a signalto the comparator 34 representative of the difference value (see alsoFIG. 2), where the difference value is compared to a first thresholdvalue. If the difference value is less than or equal to the firstthreshold value, a signal sent to the controller and in turn to thepower supply maintains the RF output current. That is, if the differencevalue is less than or equal to the threshold value, this indicates thatthe differential between the cooling factor and heating factor isrelatively low, indicating a low probability of patient burn, and noadjustments need to be made.

[0047] On the other hand, if the difference value exceeds the firstthreshold value, the value is then compared by the comparator 34 to asecond threshold (predetermined) value. The second threshold value ispreset to correspond to the situation where a patient burn is highlylikely and the RF current through the tissue needs to be terminated. Ifthe difference value exceeds the second threshold value, this indicatesthat the heating factor is too high relative to the cooling factor. Thecomparator 34 will transmit a second signal to the controller 25. Thecontroller 25 will process this signal and generate a shut off signal tothe power supply 22 to shut off the RF current. This shut off will allowthe body time to dissipate the heat and cool the tissue. However, if thedifference value exceeds the first threshold value, but does not exceedthe second threshold value, this means that although the heating factoris relatively high and the probability of patient burn likely at thecurrent power levels it is not high enough that a shut down is mandated.Instead, the output level needs to be reduced. In this circumstance, thecomparator 34 will transmit a third signal to the controller 25indicative of the high PPB value. The controller 25 in turn willtransmit a signal to the power supply 22 (see FIG. 2) to reduce theoutput power to thereby reduce the output current by a preset amount.Thus, the system remains on, but at reduced current levels, to reducethe heating affect on the tissue. As is apparent, by reducing thecurrent output, the heating factor will be reduced since the heatingfactor is calculated in part by the square of the current. By reducingthe heating factor, the differential between the heating factor andcooling factor is reduced, thus reducing the value of the probability ofpatient burn. The PPB is preferably continuously calculated in thismanner throughout the surgical procedure in order to continuouslymonitor and control the heating of tissue.

[0048] As indicated in FIGS. 2 and 3, if the probability of patient burnexceeds the first threshold value an alarm signal is sent to alarm 27 togenerate an alarm. The alarm can be in the form of a visual indicator,and audible indicator or both. Additionally, a visual and/or audiblealarm can be sounded if the PPB exceeds the second threshold valueindicating shut off of the power supply.

[0049] In summary, the following formulas explain the responses to PPBvalues:

[0050] If first threshold value<difference value and second thresholdvalue<difference value, then shut off RF output current;

[0051] If first threshold value<difference value<second threshold value,then adjust RF current supply, and

[0052] If first threshold value>difference value, then maintain RFoutput current.

[0053] It is also contemplated that if the difference value fallsbetween the first threshold value and the second threshold value, ratherthan reducing the power, the duty cycle can be reduced. This can beaccompanied by an audible or visible indicator. The duty cycle reductioncould also alternately be the first response if the PPB exceeds a firstthreshold followed by a reduction in power if the first threshold isfurther exceeded.

[0054] Both threshold values are predetermined based on the probabilityof patient burn so the overheating of tissue can be timely detected andthe electrosurgical generator adjusted accordingly. Using current valuesas part of the heating factor calculation is believed to increase theaccuracy of the PPB determination since current values are believed toactually cause the heating of the tissue.

[0055] In an alternate embodiment, the method includes the additionalstep of determining the size of the return electrode to be utilized,e.g. adult, infant, neonate, and adjusting the heating and coolingconstants accordingly. The user could inform the generator of the sizebeing used or alternatively the size can be automatically sensed by thegenerator based on the differences in the return electrode connector.

[0056] While the above description contain many specifics, thosespecifics should not be construed as limitations on the scope of thedisclosure, but merely as exemplifications of preferred embodimentsthereof These skilled in the art will envision many other possiblevariations that are within the scope and spirit of the disclosure asdefined by the claims appended hereto.

What is claimed is:
 1. A method for determining the probability of apatient burn under a return electrode in a monopolar electrosurgicalsystem comprising: calculating a heating factor adjacent the returnelectrode utilizing a first algorithm; calculating a cooling factoradjacent the return electrode utilizing a second algorithm; subtractingthe calculated cooling factor from the calculated heating factor toobtain a difference value; comparing the difference value to a thresholdvalue; and adjusting the power dependent on the relationship of thedifference value to the threshold value.
 2. The method of claim 1 ,wherein the step of calculating the cooling factor comprises the stepsof calculating the off time of the output current to obtain an off timevalue and multiplying the off time value by a first constant indicativeby the body's ability to remove heat.
 3. The method of claim 2 , whereinthe step of calculating the heating factor comprises the steps ofmultiplying the square of the output current by a second constantindicative of the measured impedance at the return electrode.
 4. Themethod of claim 3 , wherein the second constant is representative of theadherence of the return electrode to the patient.
 5. The method of claim1 , wherein the step of calculating the heating factor further comprisesthe steps of measuring the output current to obtain a first value andsquaring the first value to obtain a squared current value.
 6. Themethod of claim 5 , wherein the step of calculating the heating factorfurther comprises the steps of calculating the on time of the outputcurrent to obtain an on time value and multiplying the on time value bythe squared current value to obtain a second value.
 7. The method ofclaim 6 , wherein the step of calculating the heating factor furthercomprises the step of measuring the impedance at the return electrodeand multiplying the impedance value by the second value.
 8. The methodof claim 1 , further comprising the step of generating an alarm if thedifference value exceeds the threshold value.
 9. The method of claim 1 ,wherein the step of adjusting the power includes the step of shuttingoff the power if the difference value exceeds a predetermined value andlowering the power if the difference value is below the predeterminedvalue.
 10. The method of claim 9 , further comprising the step ofgenerating an alarm if the difference value exceeds the threshold value.11. A method for determining the probability of a patient burn in amonopolar electrosurgical system comprising: calculating a heatingfactor adjacent the return electrode utilizing a first algorithm;calculating a cooling factor adjacent the return electrode utilizing asecond algorithm; subtracting the calculated cooling factor from thecalculated heating factor to obtain a difference value; comparing thesecond value to a threshold value; and generating a warning signal ifthe difference value exceeds the threshold value.
 12. The method ofclaim 11 , wherein the step of calculating the heating factor includesthe step of multiplying a current value by a constant indicative of themeasured impedance at the return electrode.
 13. The method of claim 12 ,wherein the current value is obtained by squaring the measured outputcurrent.
 14. The method of claim 11 , wherein the first algorithmincludes multiplying a current value by the on time of the outputcurrent.
 15. The method of claim 14 , wherein the first algorithmincludes multiplying the off time of the output current by a constantindicative of the ability of the body to remove heat.
 16. Anelectrosurgical generator for use in a monopolar electrosurgical systemhaving an electrosurgical tool for treating tissue, a return electrode,and an impedance sensor in electrical communication with the returnelectrode to measure impedance of the return electrode, theelectrosurgical generator comprising: a current sensor for measuring theoutput current delivered by the generator; a microprocessor electricallyconnected to the current sensor and the impedance sensor for calculatingthe heating factor and cooling factor under the return electrode, thecalculation of the heating factor being based at least in part on themeasured output current; and a controller electrically connected to themicroprocessor for adjusting the power supply of the generator inresponse to the relationship of the calculated heating and coolingfactors.
 17. The generator of claim 16 , wherein the microprocessorincludes a first algorithm for calculating the heating factor and asecond algorithm for calculating the cooling factor.
 18. The generatorof claim 17 , wherein the first algorithm is defined as K_(c)I²t_(on)wherein K_(c) is the constant representative of the measured impedancein Ohms of the return electrode, I² is the square of the output currentin milliamps and t_(on) is the time in seconds that the output currentis delivered.
 19. The generator of claim 18 , wherein the secondalgorithm is defined as K_(h)t_(off) wherein K_(h) is the constantrepresentative of the time it takes for the body to cool down in degreesper minute and t_(off) is the time in seconds that the output current isnot being delivered.
 20. The generator of claim 18 , wherein themeasured impedance is indicative of the degree of adherence to thepatient of the return electrode.
 21. The electrosurgical generator ofclaim 16 , wherein the microprocessor includes an algorithm forsubtracting the cooling factor from the heating factor to calculate adifference value, and the generator further comprises a comparatorelectrically connected to the microprocessor for comparing thedifference value to a threshold value, the comparator being electricallyconnected to the controller to generate a first signal indicative of therelationship of the difference value and the predetermined value. 22.The electrosurgical generator of claim 21 , further comprising an alarmelectrically connected to the comparator for generating a warning signalif the difference value exceeds the threshold value by a predeterminedamount.
 23. The electrosurgical generator of claim 22 , wherein thecontroller generates a shut off signal to terminate power if thedifference value exceeds a predetermined value, the predetermined valuebeing greater than the threshold value.
 24. The electrosurgicalgenerator of claim 23 , wherein the controller generates a second signalto reduce the power if the difference value exceeds the threshold valuebut does not exceed the predetermined value.